1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor for detecting a torque based on a change in magnetic characteristics due to magnetostriction, and an electric power steering apparatus including such a magnetostrictive torque sensor.
2. Description of the Related Art
One known contactless torque sensor is a magnetostrictive torque sensor for detecting a torque based on a change in magnetic characteristics due to magnetostriction. The magnetostrictive torque sensor is used to detect the steering torque of automotive steering apparatus (see Japanese Laid-Open Patent Publication No. 2005-321316).
The magnetostrictive torque sensor of the type described above comprises two magnetostrictive films having different magnetic anisotropic properties and mounted on a rotatable shaft and detecting coils disposed in confronting relation to the respective magnetostrictive films. When a torque is applied to the rotatable shaft to twist the rotatable shaft, the magnetic permeability of the magnetostrictive films changes, thereby changing the inductance of the detecting coils.
The detecting coils are intermittently excited through respective switching devices in constant exciting periods. The magnetostrictive torque sensor intermittently detects the torque based on the difference between the detected output signals from the respective detecting coils when the excitation of the detecting coils is finished, i.e., at the ends of the exciting periods. Between adjacent ones of the exciting periods, i.e., between the end of an exciting period and the start of a next exciting period, the energy stored in the inductance of the detecting coils is discharged through resistors.
A driving and detecting circuit of a detecting coil has a specific arrangement shown in FIG. 8 of the accompanying drawings. As shown in FIG. 8, a resistor having a resistance R and a detecting coil having a inductance L are connected in series to a power supply having a voltage E. A transistor Q for switching is connected to the series-connected circuit.
When a switching signal Ss shown in FIG. 9A of the accompanying drawings is supplied to the base of the transistor Q, the detecting coil is excited in exciting periods between time t1 and time t2, time t3 and time t4, and time t5 and time t6.
A change in the voltage of a detected output signal Sc in each of the exciting periods is expressed by the following equation (1):V(t)=E·exp(−Rt/L)  (1)where E represents the voltage of the power supply for excitation, V(t) the voltage at the end of each of the exciting periods, R the resistance of the resistor through which the voltage E is applied to the detecting coil, and L the inductance of the detecting coil.
It has also been proposed to detect a failure of the magnetostrictive torque sensor based on the sum of the detected output signals from the detecting coils (see Japanese Laid-Open Patent Publication No. 2006-064445).
Specifically, Japanese Laid-Open Patent Publication No. 2006-064445 discloses a magnetostrictive torque sensor including additional two detecting coils and two switching devices for allowing the magnetostrictive torque sensor to detect a torque and a failure regardless of a temperature change and a magnetic field change.
Furthermore, Japanese Laid-Open Patent Publication No. 2007-309925 reveals a magnetostrictive torque sensor including four detecting coils in the form of two parallel-connected arms each comprising two series-connected detecting coils. The magnetostrictive torque sensor has a bridge circuit made up of four FETs as switching devices for applying a voltage between the opposite ends of the two detecting coils of each arm while changing the direction of an exciting current (energizing current) in each switching period.
The RL circuit shown in FIG. 8 which is disclosed in Japanese Laid-Open Patent Publication No. 2005-321316 detects a voltage change (transient response voltage) V(t). If a stray resistance occurs between the collector terminal of the transistor Q and the power supply, or such a stray resistance varies, the detected output signal Sc according to the equation (1) changes from a solid characteristic curve shown in FIG. 9B of the accompanying drawings to a dashed characteristic curve shown in FIG. 9B. At this time, it is difficult for the magnetostrictive torque sensor to detect a torque variation stably. Actually, in an automotive steering apparatus, the detecting coils are mounted on the side of the steering shaft and the transistor Q and the power supply are included in an electronic circuit. Therefore, connectors or couplers are generally interposed between the power supply and the series-connected circuit of the resistor and the detecting coil and between the series-connected circuit and the collector terminal of the transistor Q. When the contact resistance (i.e., the above stray resistance) of the connectors changes, the transient response voltage tends to change.
The torque sensor disclosed in Japanese Laid-Open Patent Publication No. 2005-321316 is unable to determine whether a change in the transient response voltage is a desired change to be detected which is based on a change in the inductance that is caused when the steering shaft is twisted, or an increased error caused by a change in the contact resistance of the connectors.
According to the magnetostrictive torque sensor disclosed in Japanese Laid-Open Patent Publication No. 2007-309925, the detected voltage V at the midpoint between the two series-connected detecting coils of each arm is produced as a divided voltage (see the equation (2) below) between impedances Z1, Z2 of the respective inductances L of the series-connected detecting coils to which the power supply voltage E is applied. The divided voltages appearing at the midpoints of the respective arms are detected by a differential amplifier. Even though the connectors are interposed between the four detecting coils and the electronic circuit, the detected voltage (divided voltage) V is essentially not susceptible to a change in the resistances of the connectors.
According to the magnetostrictive torque sensor disclosed in Japanese Laid-Open Patent Publication No. 2007-309925, however, since the bridge circuit of the four FETs is used to energize the four detecting coils, the circuit arrangement is complex with the increased number of wires or interconnects between the electronic circuit including the bridge circuit and the four detecting coils.Detected voltage (divided voltage) V=E×Z2/(Z1+Z2)  (2)